Up to now, a technique has been known in which the position of the portable device is identified by receiving the radio waves transmitted from the portable device in response to the radio waves transmitted from the transmitting antennas installed on the multiple places of the vehicle. For example, Patent Literature 1 discloses that the position of the portable device relative to the vehicle is identified according to which of the transmitting antennas installed on the multiple locations of the vehicle the portable device responds to the radio waves transmitted from.
However, in the technique disclosed in Patent Literature 1, in order to improve a precision in the position identification of the portable device, there is a need to increase the number of transmitting antennas, and further finely implement the adjustment of output strengths of the respective transmitting antennas to prevent the outputs of the respective transmitting antennas from interfering with each other. This results in such problems that it is not easy to enhance the precision in the position identification of the portable device, and the costs increase.